The present invention relates to techniques of driving a motor with a frequency converter.
“Influence of Insulation of General Motor Driven with 400 V Class Inverter”, the Japan Electrical Manufacturers' Association, a corporate juridical person, 1995, March, has the following description that “While a motor is driven with an inverter, a surge voltage generated by switching of the inverter is superposed upon an output voltage of the inverter. If this surge voltage is high, insulation of the motor is adversely affected, resulting in a broken case. This document describes the inverter surge voltage generation mechanism and its countermeasure in order to avoid beforehand such a case (an excerpt from p. 1, ll. 2 to 6).”
This document further gives the following methods as a countermeasure for insulation damages by a surge voltage during driving a motor with a 400 V class inverter (an excerpt from p. 3, ll. 2 and 3):
“4.2 Surge Voltage Suppressing Method
There are a voltage rise suppressing method and a peak value suppressing method in order to suppress a surge voltage.
Output Reactor
If a wiring length is relatively short, a surge voltage can be lowered by suppressing a voltage rise (dv/dt) by installing AC reactors on the output side of an inverter (refer to FIG. 4(1)).
However, if the wiring length becomes long, it becomes difficult to suppress the peak value of a surge voltage in some cases.
Output Filter
A peak value of a terminal voltage of a motor is suppressed by installing filters on the output side of an inverter (refer to FIG. 4(2)) (an excerpt from p. 3, ll. 6 to 15).”
FIG. 4 in the indications in parentheses (refer to FIG. 4(1) and FIG. 4(2)) in the excerpts is the drawing in the document and does not concern FIG. 4 in this specification.